Composite materials comprising a reinforcement matrix which is coated or in the alternative impregnated with a polymer resin are well known to the art. In many instances, it is desirable to partially cure the polymer resin of the composite material subsequent to the coating or impregnation step in order to form a "prepreg" which possesses desirable rheological properties for easy handling during subsequent processing, for example in formation of a final laminated or structural form. Examples of such materials include those described in U.S. Pat. Nos. 4,414,264, 4,372,347 as well as others.
In the process of producing such composite materials containing a polymer resin, a web formed from fibers, fabric, paper, canvas, or the like of glass, quartz, graphite and/or aromatic polyamide as well as of cellulostic materials in a flexible or fiber form is contacted with one or more polymer resins so to coated and/or impregnate the web which is then subjected to a curing operation wherein at least part of the polymer resin is partially cured. This web impregnated with a polymer resin which has been partially cured and/or dried is commonly referred to in the art as a "prepreg" and this term will be interchageably used to also mean the composite material irregardless of the level of cure of the resin material. Afterwards, the web containing the partially cured polymer resin, or prepreg, may be cut into pieces, layered in register to form a structure which comprises a plurality of layers, and then subjected to further processing wherein the structure is laminated and further curing of the polymer resin is achieved. In many instances, one or more of the layers of the prepreg is imparted with a thin layer of an electrically conducting structure, for example copper, invar copper, aluminum, silver, gold in a foil form prior to, during, or after the ultimate lamination of the layers in order to form a circuit board as is widely used in electronic and/or electrical devices.
An important requirement of the fabrication process of any laminate structure formed from prepregs is the determination of the amount, or degree of cure of the polymer resin, and methods for determining this information have been developed and are known to the art.
Several test methods are known in the art for determining the degree of cure of a resin contained in a composite material. For example, one commonly employed method for determining the degree of cure of a prepreg resin comprises the so called "gel time test", alternately known as the "dry rubber test" or the "tack test". This method involves removing resin from the prepreg and measuring the time required for gelation of the resin to occur at an elevated temperature. Various instruments are used to measure the gelation point. In the electrical laminating industry, this method is performed in accordance with IPC (Institute of Printed Circuits) Test Method No. 2.3.18, which employs a hot platen at 171 deg. C. Other gel tests involve powdering the resin from the composite material and then stirring the powder resin on a hot plate until gelation occurs. One such procedure is that described in U.S. Military Specification MILP-13949 Revision H. These gel time tests are disadvantageous in that they require off-line testing of the composite material and the additional, and often difficult step of separating resin from the composite material. Gel time tests are also disadvantageous in that they are subjective with regard to operator technique and consequent suffer variation in the determination of the gelation point. Further variations which may manifest themselves are due to variations in the platen temperature, as well as variance in the characteristics of the air flow over the sample which affects the rate of cure. Generally, gel tests require from one to two minutes to remove and collect resin from the prepreg or the composite material, and three to ten minutes to measure the gel point and prepare the equipment for the next test. When testing resins having a low degree of cure, an even greater time is required to measure the gelation point.
Another method for determining the degree of cure of a resin in a composite material such as a prepreg is the flow testing method. This method involves measuring the amount of resin which flows out of a fixed number of plies of a prepreg during lamination in a small press. In the electronics industry, this method is generally performed in accordance with IPC Test Method No. 2.3.17 wherein a number of measured samples are cut from the prepreg product, weighed and placed between release forms in a hydraulic press. The samples are pressed at an elevated temperature for ten minutes and them removed from the press. After cooling, a known area is cut from the samples and weighed. The percent flow, which is an indication of the degree of cure, is calculated from the difference in weight per area between the original samples and the pressed samples. The flow testing method is disadvantageous in that it requires at least ten minutes during the pressing portion, and subsequent to pressing requires approximately ten additional minutes for analytical evaluation of the sample. The prepreg product which is tested is destroyed, and the results are determinative of the resin content of the prepreg. This method is also disadvantageous in that operator error can easily occur in one or more of the several handling steps. Such variations and errors my be inconsistencies in the press temperature, the deniations from even distribution of pressure across the surface of the platens, i.e., their "trueness".
A further test which is known for measuring the degree of cure in a composite material is the rheology test method. This method involves measuring the change in viscosity over time of the prepreg resin during cure at elevated temperatures. As in the gel time test, resin must be separated from the prepreg before testing. Once separated, a resin sample is placed in or on an instrument which can measure the change in viscosity over time by measuring the increase in stress during shear of the sample as it cures. This method is based on the property that resins with higher degrees of cure have higher viscosity. Generally, the instrument employed comprises a cone and plate viscometer or parallel plate viscometer, both of which are commercially available. While this test method produces very accurate results, it is disadvantageous in that it requires ten to twenty minutes per sample test and the choice of the shear rate and test temperature can significantly influence the results of the test and its reproducibility.
One technique described in U.S. Pat. No. 4,874,948 to Cielo describes a method and an apparatus which utilizes a laser light source for heating a portion of a polymeric composite material, means for monitoring the temperature fluctuations of the heated surface portion and processing means for utilizing data obtained from the monitoring means for comparing the data with a calibration reference and subsequently providing a measure of the degree of cure of the polymeric composite. This method however materially alters the sample being tested, and while effective in providing a measure of the degree of cure of the surface of a sample does not provide a measure of the degree of cure of the total cross-section of a sample.
A further U.S. Pat. No. 4,582,520 to Sturm describes a system for evaluating the degree of cure of a carbonaceous material web which is used for controlling the operation of an apparatus for producing a fiberglass web used as an insulation material, wherein the system utilizes an infrared radiation apparatus having a plurality of filters wherein limited frequencies may be examined and evaluated.
U.S. Pat. No. 4,798,954 to Stevenson describes a system for monitoring the degree of cure of a resin in a molding press or other molding device.
Several other analytical techniques have also been developed to measure the state or degree of cure of a material including thermal analysis, mechanical analysis, differential scanning calorimetry and dielectric analysis. However, these methods also suffer from one or more of the disadvantages of the method discussed above. For example, both the thermal and dielectric analysis method require significant amounts of time and result in the destruction of the prepreg materials which are tested.
Still further, these methods of analysis generally require physical testing of the polymer prepreg which at least requires that any tested polymer prepreg be removed from a production process, and tested. The analytical methods known to the art are inadequate for use in a production process whereby the degree of cure of the polymer being produced might be utilized in controlling production in that they are (a) time consuming, and (b) frequently require the destruction of at least part of the prepreg being produced.
Accordingly, there exists a continuing need in the art for new and improved methods for the determination of the degree of cure of a polymer resin containing prepreg which is non-destructive and faster than many known methods. A further need is for new and improved methods for the production of composite materials which comprise a polymer resin which is at least partially cured during the production process.